Switch to using cargo-metadata

This both makes the crate compile way faster and avoids having to touch
tons of unstable, undocumented cargo internals. This requires Rust 1.41
(the current beta) for some cargo-metadata features.

There are some subtle differences in the new output (particularly in
handling dependencies between crates in workspaces), but they're pretty
minor.

Closes #73
Closes #69
Closes #66
Closes #65
Closes #63
Closes #61
Closes #57
Closes #20

.circleci/config.yml

@@ -1,39 +1,23 @@
-deps_key: &DEPS_KEY
-  key: dependencies-1.40-{{ checksum "Cargo.lock" }}
-
-restore_registry: &RESTORE_REGISTRY
-  restore_cache:
-    key: registry
-
-save_registry: &SAVE_REGISTRY
-  save_cache:
-    key: registry-{{ .BuildNum }}
-    paths:
-      - ~/.cargo/registry/index
-
-restore_deps: &RESTORE_DEPS
-  restore_cache:
-    <<: *DEPS_KEY
-
-save_deps: &SAVE_DEPS
-  save_cache:
-    <<: *DEPS_KEY
-    paths:
-      - target
-      - ~/.cargo/registry/cache
-
 version: 2
 jobs:
   build:
     docker:
-      - image: rust:1.40.0
+      - image: rustlang/rust:nightly
     steps:
       - checkout
-      - run: apt-get update
-      - run: apt-get install -y --no-install-recommends cmake zlib1g-dev
-      - <<: *RESTORE_REGISTRY
-      - <<: *RESTORE_DEPS
+      - run: rustc --version > ~/rust-version
+      - restore_cache:
+          key: registry
+      - restore_cache:
+          key: dependencies-{{ checksum "~/rust-version" }}-{{ checksum "Cargo.lock" }}
       - run: cargo build --locked
       - run: cargo run --locked -- tree
-      - <<: *SAVE_REGISTRY
-      - <<: *SAVE_DEPS
+      - save_cache:
+          key: registry-{{ .BuildNum }}
+          paths:
+            - /usr/local/cargo/registry/index
+      - save_cache:
+          key: dependencies-{{ checksum "~/rust-version" }}-{{ checksum "Cargo.lock" }}
+          paths:
+            - target
+            - /usr/local/cargo/registry/cache